1. Field of the Invention
The present invention relates to a camera having a member which is reciprocally moved to a photographing position each time a photographing operation is performed like a photographing lens in, e.g., an auto-focus camera.
2. Description of the Prior Art
An auto-focus camera comprises a cylindrical cam member having a plurality of continuous stepped teeth, i.e., a so-called lens positioning member, and an actuating member having grooved cams engaged with a photographing lens, i.e., a so-called a lens drive member as members for moving a photographing lens to an in-focus position, and regulating it at that position during exposure. In a positioning method of the auto-focus camera, a rotational position of the lens positioning member is first determined based on distance data from a distance measurement device. The lens drive member is then rotated to move the photographing lens along an optical axis by leads of the grooved cams, so that the photographing lens is caused to abut against a predetermined stepped tooth of the lens positioning member to be positioned at a photographing position.
The lens positioning and drive members must be operated over wider ranges as the camera has higher performance. Meanwhile, a high-speed focusing operation is required so as not to lose "shutter chance". Therefore, an elastic operation obtained by releasing a biased spring member in a conventional camera cannot provide constant drive power, and a capacity itself is also small. In recent cameras, in place of this conventional mechanism, the lens positioning and drive members are driven by utilizing high-speed, constant drive power of a motor incorporated in the camera.
As a result, even for a camera having a large aperture ratio and a relatively large focal length which requires high focusing precision and hence is considered to be difficult to have an automatic focusing function, a camera comprising an automatic focusing apparatus can be realized.
A photographing lens used in an auto-focus camera is linearly guided to move forward or backward along the optical axis such that a plurality of pins project from an outer periphery of a lens frame, and are engaged with straight grooves of a support member fixed to a lens barrel.
At the same time, the pins are also engaged with the grooved cams of the lens drive member. When the lens drive member is rotated in the circumferential direction, the pins are pushed by the corresponding inclined surfaces of the grooved cams so as to be moved along the optical axis and to abut against the stepped teeth. In this case, the photographing lens must be satisfactorily pressed by the predetermined stepped tooth to occupy an in-focus position. For this purpose, the lens drive member must be rotated to overcome the frictional resistance with the pins. Furthermore, the photographing lens must be held at the predetermined stepped tooth position until exposure is completed. Thus, the photographing lens must be biased in the rotational direction for a predetermined period of time to be satisfactorily fixed.
For this purpose, a strong motor capable of generating a large torque is used.
However, when the photographing operation is completed and the photographing lens is returned to an initial position, since the photographing lens is satisfactorily fixed to the predetermined stepped tooth by the frictional resistance between the pins of the photographing lens and the grooved cams in an initial process, a load is large, and the motor requires a large torque even when it is rotated in a reverse direction so as to start the lens drive member which is in a stop state. However, when the photographing lens begins to be disengaged from the stepped tooth after this process, the load of the lens drive member is immediately decreased.
As a result, the return speeds of the photographing lens, the lens drive member, and the lens positioning member following the lens drive member are increased. When the photographing lens reaches the initial position and the respective members are simultaneously stopped, a large shock is generated, and is applied to a power transmission system connected to the motor as well as to the respective members.
In general, a motor incorporated in a camera is placed at a position slightly separated from the photographing lens in terms of an installation space, and is coupled to the lens drive and positioning members through a plurality of reduction gear trains. The power transmission system includes many connecting portions, and shafts are easy to flex. In addition, since a gear of a small module is used and the number of teeth cannot be increased, the gears are not resistant to the above-mentioned shock. That is, the gears may be worn, damaged, or cause a so-called tooth lock state wherein the tooth tip of the drive gear abuts against that of the driven gear so as not to be rotated in neither forward nor reverse direction.